[Travis] Anotehr fix attempt

.travis.yml

@@ -30,7 +30,8 @@ before_install:
   - sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
 
   # install 3rd party boards
-  - additionalUrls="boardsmanager.additional.urls=\
+  - |
+    additionalUrls="boardsmanager.additional.urls=\
     http://arduino.esp8266.com/stable/package_esp8266com_index.json,\
     https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json,\
     https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json,\